1. Field of the Invention
The present invention relates to the field of integrated electronic circuit technology. More particularly, the invention relates to a reliable and manufacturable capacitor-like, electrically-programmable interconnect device to be used in integrated circuits.
2. The Prior Art
Integrated electronic circuits are usually made with all internal connections set during the manufacturing process. However, because of high development costs, long lead times, and high manufacturing tooling costs of such circuits, users often desire circuits which can be configured or programmed in the field. Such circuits are called programmable circuits and they usually contain programmable links. Programmable links are electrical interconnects which are either broken or created at selected electronic nodes by the user after the integrated device has been fabricated and packaged in order to inactivate or activate respectively the selected electronic nodes.
Programmable links which are open circuits until a current path is created by a user are called antifuses. Antifuses have been used in numerous types of user-programmable circuits. Antifuses typically consist of two electrodes formed from conductor and/or semiconductor materials having some kind of a dielectric or insulating material between them. During programing, the dielectric material at selected points in between the conductive materials is broken down by predetermined applied voltages, thereby electrically connecting the conducting or semiconducting materials together.
Various materials have been suggested for both the antifuse electrodes and the dielectric or insulating layers. Some of these suggested dielectric materials require a relatively high current and voltage during programming, require complex manufacturing techniques and have low reliability during programming because it is difficult to control the reproducability of the conductive state due to the nature of the materials involved. In addition, the programming process results in a link having a finite resistance in the order of several hundred to several thousand ohms. This characteristic of some known antifuse elements renders them relatively unsuitable for use in high speed circuits.
Lately, there has been much interest in antifuse elements disposed in layers above and insulated from the surface of the silicon substrate in an integrated circuit. Of particular interest in this regard are antifuse structures for use between two metal interconnect layers in integrated circuit structures. When the lower electrode for an antifuse is to be formed in layers above the semiconductor or other substrate, it is usually fabricated from a metal layer or a composite layer which may include a first metal layer and another layer such as silicon or another metal or barrier layer.
Two problems which must be addressed when fabricating an antifuse are the problem of assuring reliability and the separate-but-related problem of assuring manufacturability of many antifuses in a production environment. When the lower electrode of the antifuse is located in a layer above the surface of the substrate, such as a metal layer, planarity of the upper surface of the lower electrode becomes an issue. If the surface of the lower electrode is sufficiently non-planar, reliability and manufacturability problems may arise because the thickness of the antifuse dielectric, which determines programming voltage, and programming reliability margins, may not be controllable enough to allow design of a manufacturable and reliable product.
For example, if the lower electrode of an antifuse is fabricated from a layer of aluminum-silicon-copper (AlSiCu) alloy with various Si and Cu concentrations, its upper surface will be characterized by non-planar hillock formations as a result of its fabrication. If the hillocks are high enough, they will cause difficulties in selecting the thickness of the dielectric layer which will cover them. In addition, the tips of the hillocks will be sources of concentrated electric fields which may result in unpredictable and undesired programming of individual antifuses. In a typical metallization process, hillocks may range in height from about 0.5 to 2.0 microns and have fairly sharp tips, resulting in considerable and unpredictable field concentrations at the tips of the hillocks during programming.
A preferable solution to this problem would be to provide a structure which avoids the problems inherent in antifuse structures incorporating hillock formations but allows standard metallization techniques and materials to be employed in the antifuse fabrication process. A preferable antifuse structure would also exhibit a minimum capacitance in its unprogrammed state.